Color video camera with auto-white balance control

ABSTRACT

In a color video camera providing color video signal components corresponding to a light image projected on an image pickup, only the levels of the color video signals components which correspond to at least substantially white regions of the light image are detected, and such detected levels of the color video signal components are integrated so as to obtain average values thereof as white-balance detection signals. The color video signal components which correspond to at least substantially white regions of the light image, and thus have their levels detected, may be those which occur during intervals corresponding to a white-balance detecting area positioned to correspond with a selected white area of the light image. Alternatively, the color video signal components may be sampled to provide pixel-by-pixel values thereof, whereupon, only those pixel-by-pixel values which are determined to be within a predetermined range of white as defined by a black body radiation curve are subjected to integration. The predetermined range is made relatively wide when the valve of the color video signal components deviate substantially from a desired white-balance relationship and the range is reduced as the values of the conponents come near to the desired relationship.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to color video cameras and, more particularly, is directed to the auto-white balance control of such cameras.

2. DESCRIPTION OF THE PRIOR ART

Color video cameras require adjustment of the white balance thereof in dependence on the color temperature of the source of light illuminating a scene in the field of view of the camera. Manual white-balance adjustment has been conventionally performed by placing a white object in the field of view of the camera, for example, by mounting a white cap on the lens, and then setting the gains of the several color signals so as to establish a predetermined ratio among such color signals. However, manual white-balance adjustment requires readjustment of the white-balance whenever the light source changes, and thus is bothersome. On the other hand, a color video camera having a full automatic white-balance control system can automatically adjust the gains of the several color signals whenever the color of the light source is changed without any intervention by the camera operator. However, the conventional fully automatic white-balance control system may not achieve precise white-balance adjustment, and thus is not reliable in operation. More specifically, the conventional fully automatic white-balance control system operates on the premise that all colors are uniformly distributed in the picture or scene in the field of view of a camera so that summing all of the colors in the picture should result in white. Based on such premise, the conventional fully automatic white-balance control system sets the gains of the several color signals with reference to the sum or integral of the color signals for the entire picture which is presumed to be white. However, the several colors are not distributed uniformly in the picture so that the sum or integral of the color signals does not substantially correspond to white unless the picture itself contains a fairly large white area. Therefore, the conventional fully automatic white balance control system cannot attain accurate white-balance adjustment for all pictures or scenes in the field of view of the camera.

By reason of the foregoing, a color video camera having a fully automatic white-balance control system, as aforesaid, has been further provided with a manual white-balance adjustment for use in achieving accurate white-balance adjustment in those cases where, by reason of the nature of the scene or picture in the field of view of the camera, accurate white-balance adjustment by the fully automatic system is not to be expected. In such case, of course, the manual white-balance adjustment requires the placement of a white cap on the lens, as previously described, so that the use of the manual white-balance adjustment is inconvenient.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide a color video camera with a white-balance control system which is conveniently operable to achieve accurate white-balance adjustment.

In accordance with an aspect of this invention, in a color video camera providing color video signal components corresponding to a light image projected onto a light reception surface of an image pickup, there are provided means for detecting levels of the color video signal components which correspond only to at least substantially white regions of the light image, means for integrating the detected levels of color video signal components so as to obtain average values thereof, and means for effecting white-balance control with reference to said average values of the color video signal components.

In one embodiment of the invention, means are provided for defining a white-balance detecting area which can be positioned to correspond substantially with a selected area of the light image which is at least substantially white, and the color video signal components are supplied to respective integrators only during intervals of the color video signal components which correspond to the defined white balance detecting area.

In another embodiment of the invention, the color video signal components are sampled to provide pixel-by-pixel values thereof, and only those pixel-by-pixel values are integrated which are determined to be within a predetermined range of white as defined by a black body radiation curve. It is a further feature of this embodiment to make the mentioned range relatively wide when the values of the color video signal components deviate substantially from a relationship characteristic of a desired white-balance, and to narrow such range as the values of the color video signals components near the relationship characteristic of the desired white-balance.

The above, and other objects and advantages of the invention, will be apparent in the following detailed description of an illustrative embodiment which is to be read in connection with the accompanying drawings forming a part hereof, and in which corresponding parts and components are identified by the same reference numerals in the several views of the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a video camera of a type that may desirably embody the present invention;

FIG. 2 is a schematic representation of a lens system which is included in the camera of FIG. 1;

FIG. 3 is a schematic representation of a complementary-colored checkerboard pattern that may be used with an image pickup device of the video camera shown in FIG. 1;

FIGS. 4A and 4B together constitute a block diagram of an optical detector included in the video camera of FIG. 1 in accordance with one embodiment of the present invention;

FIGS. 5A and 5B are diagrammatic views to which reference will be made in explaining how the optical detector shown in FIG. 4 separates luminance and chrominance components from the video signal produced by the checkerboard pattern shown in FIG. 3;

FIG. 6 is a graphical representation of the frequency characteristics of digital high pass filters used in the optical detector of FIG. 4;

FIG. 7 is a graphical representation of the spectral characteristics of the digital filter circuits as a function of the focus condition of the lens included in the video camera of FIG. 1;

FIGS. 8A and 8B are waveform diagrams to which reference will be made in explaining the function of coring circuits included in the optical detector of FIG. 4;

FIGS. 9A-9D are waveform diagrams to which reference will be made in explaining another feature of the optical detector of FIG. 4;

FIG. 10 is a partial block, partial schematic diagram of a portion of the optical detector shown in FIG. 4;

FIGS. 11A and 11B represent predetermined focus detection areas of the picture derived from the video camera of FIG. 1;

FIG. 12 is a graphical representation of the transfer characteristic of a circuit used for auto-exposure control in the optical detector of FIG. 4;

FIGS. 13A and 13B are diagrammatic views to which reference will be made in explaining auto-exposure control when a scene being imaged is back-lighted;

FIG. 14 is a schematic representation of a circuit that may be used with the auto-exposure control portion of the optical detector shown in FIG. 4;

FIGS. 15A and 15B represent the manner in which the transfer characteristic shown in FIG. 12 may be varied for different lighting conditions;

FIGS. 16A and 16B are diagrammatic views to which reference will be made in explaining the operation of a white balance detector included in the optical detector of FIG. 4 in accordance with an embodiment of this invention;

FIG. 17 is a block diagram showing a video camera with an automatic white balance control system according to another embodiment of this invention; and

FIG. 18 is a graphic representation of a black body radiation curve, and to which reference will be made in explaining operation of the white-balance control system of FIG. 17.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring initially to FIG. 1, it will be seen that a video camera with which the present invention may be used is there shown to be similar to that described in application Ser. No. 07/393,804, filed Aug. 15, 1989, now U.S. Pat. No. 4,998,162, granted Mar. 5, 1991. Such video camera generally includes a lens system 1 which preferably includes zoom lens elements and an adjustable lens group for focusing the lens system between near and far (infinity) focus conditions. As referred to herein, the expression "just-focused" means that the lens system is properly focused on an object being imaged. Lens system 1 is illustrated in FIG. 2 to be comprised of lens groups F1, F2, F3 and F4, wherein lens group F2 constitutes a zoom lens arrangement and lens group F4 constitutes a focus lens system. A PN filter 17 is disposed between lens groups F2 and F3, and an adjustable aperture 18, such as an adjustable iris, is illustrated adjacent the PN filter for achieving exposure control. An infrared inhibitor 19 serves to remove infrared radiation from the optical image which is focused by the lens system 1 onto an imaging device 2 (FIG. 1).

Lens group F4 is driven by a focus motor 3 (FIG. 1). Iris 18 is driven by an iris motor 4 to increase or decrease the size of the aperture and, thus, the exposure of lens system 1. The focus and iris motors 3 and 4 are controlled by a system controller 12 through driver circuits 13 and 14, respectively. The system controller 12 receives signals from an iris position detector 5 and a zoom position detector 6 which respectively indicate the positions of the iris 18 and the zoom lens group F2, respectively. The manner in which the system controller 12 operates to adjust the iris 18 and the zoom lens group F4 will be later described.

The imaging device 2, which is preferably, comprised of a CCD array of the type shown in FIG. 3, is adapted to generate a video signal corresponding to an optical image projected thereon by the lens system 1 as the individual elements which comprise the CCD array are scanned. As shown in FIG. 3, the CCD array is comprised of rows and columns of pixel elements arranged in a complementary-colored checkerboard pattern. This pattern is comprised of successive rows or lines L1, L2, L1, L3, L1, L2, etc., wherein each line L1 is formed of alternating cyan and yellow pixel elements Cy, Ye, Cy, Ye, etc., each line L2 is comprised of alternating green and magenta pixel elements G, M, G, M, etc. and each line L3 is comprised of alternating magenta and green elements M, G, M, G, etc. In one embodiment, each row of CCD image pickup device 2 is comprised of 510 pixel elements; and in another embodiment, each row is comprised of 760 pixel elements. The pixel elements in a row are scanned successively on a row-by-row basis. For the embodiment in which a line is comprised of 510 pixel elements, the pixel elements are scanned at a scanning clock frequency (8/3)f_(sc) =9.55 MHz. In the alternative embodiment in which each line is comprised of 760 pixel elements, the successive pixel elements are scanned at a scanning clock frequency 4f_(sc) =14.32 MHz.

Returning to FIG. 1, each scanned pixel element produces a pixel signal level of a magnitude determined by the brightness of the respective point in the image projected on imaging device 2. Successive pixel levels in the signal output of the device 2 are sampled by a sample-and-hold circuit 7 and each sample is converted to digital form by an A/D converter 9 after passing through an automatic gain control (AGC) circuit 8. The AGC circuit 8 preferably is an analog circuit supplied with a gain setting voltage from system controller 12. The system controller 12 includes a microprocessor and produces a digital gain controlling signal which is converted to analog form by a D/A converter 15.

As will be discussed below in conjunction with FIGS. 5A and 5B, the complementary colored checkerboard pattern comprising CCD pickup device 2 is scanned or read so that each scan output is comprised of the sum of the pixel level of two vertically adjacent pixel elements, that is, two adjacent rows are read simultaneously. For example, a pixel element in row L1 and a vertically aligned pixel element in row L2 are scanned together, followed by the next pixel element in row L1 and a vertically adjacent or aligned pixel element in row L2. Similarly, when adjacent rows L1 and L3 are scanned together, one pixel element in row L1 is scanned concurrently with a vertically aligned pixel element in row L3. These vertically adjacent pixels are summed and the result thereof is sampled by the sample-and-hold circuit 7.

A/D converter 9 preferably produces a 10-bit digital signal representing each sample produced by the sample-and-hold circuit 7. The digitized samples are supplied from A/D converter 9 to digital video processor 10 and also to an optical detector 11. The optical detector 11 generally includes an auto-focus (AF) detection circuit, an auto-exposure (AE) detection circuit and an auto- white balance (AWB) detection circuit, which will be described in greater detail in conjunction with FIG. 4. The AF, AE and AWB detection circuits produce auto-focus, auto-exposure and auto-white balance detection signals, respectively. These detection signals are supplied from optical detector 11 to system controller 12 whereat they are used to control driver circuit 13 for the focus motor 3 so as to adjust the focus condition of lens system 1, to control driver circuit 14 for the iris motor 4 and thereby vary the exposure aperture of iris 18 so as to control the exposure condition of the video camera, and to control the digital video processor 10 in accordance with the detected white balance of the image picked up by the video camera. System controller 12 also produces the gain setting signal for the AGC circuit 8 in response to the detected exposure level sensed by the optical detector 11.

Optical detector 11 and system controller 12 are interconnected by way of a serial interface, whereby the AF detection signal, AE detection signal and AWB detection signal produced by optical detector 11 are supplied as AF, AE and AWB control signals by way of a serial output port from the optical detector 11 to the system controller 12; and whereby control signals produced by system controller 12 are applied to the optical detector 11 by way of a serial input port. It is appreciated that a serial interface reduces the complexity, bulk and weight of the interconnections between the optical detector 11 and the system controller 12. Preferably, the signals communicated between the optical detector and the system controller exhibit a periodicity corresponding to the vertical period intervals of the video signal.

Digital video signal processor 10 produces luminance and chrominance signals corresponding to the digitized samples supplied thereto by A/D converter 9. The manner in which the digital video processor 10 operates forms no part of the present invention per se and, therefore, is not further described herein. Suffice it to say that the digitized luminance and chrominance signals produced by the digital video processor are converted to analog form by D/A converters 15a and 15b, respectively. As a result, analog luminance and chrominance signals are supplied to output terminals 16a and 16b, respectively, from which they may be recorded, displayed or processed further.

Referring now to FIG. 4, there is illustrated a more detailed block diagram of the optical detector 11 which is shown to comprise an auto-focus detector 21, an auto- exposure detector 22 and an auto-white balance detector 23, delineated within broken lines. Each of these detectors now is described in greater detail.

AUTO-FOCUS DETECTOR 21

Digital circuits may be used to implement the auto-focus detector 21, as illustrated in FIGS. 4A and 4B. Alternatively, the auto-focus detector may be implemented by suitable programming of a microprocessor, as will become apparent from the following description.

In the case where the auto-focus detector 21 is implemented by digital circuits, the detector 21 is generally comprised of digital filters which are further described below and are adapted to filter intermediate and higher frequency components from a video signal, selector circuits adapted to select different filtering characteristics from the digital filters as lens group F4 is adjusted to its just-focused position, coring circuits adapted to reject noise signals from the filtered intermediate and higher frequency components, gate circuits adapted to limit the filtered higher frequency components to discrete auto-focus areas of the imaged picture, peak detectors adapted to detect the peaks of the intermediate and higher frequency components derived by the digital filters, and integrators adapted to integrate the detected peaks.

As shown more particularly in FIGS. 4A and 4B, an input terminal 31 receives a digitized composite video signal, as may be produced by the A/D converter 9 of FIG. 1. A luminance separator 32 and a chrominance separator 33 are coupled to input terminal 31 to separate a luminance component Y and chrominance component C_(R) and C_(B) from the digitized composite video signal. An auto-focus signal which represents the focus condition of lens system 1 is derived from the separated luminance component Y in a discrete area of the video picture being imaged. This discrete area is established by an auto-focus area setting circuit 24 which establishes the focus detection area in which intermediate and higher frequency components of the luminance signal are examined. The focus detection area is established by system controller 12 which supplies a suitable control signal to auto-focus area setting circuit 24 by way of a serial input port 28. For example, since the complementary colored checkerboard array of pixel elements shown in FIG. 3 is scanned pixel by pixel and line by line, the auto-focus area setting circuit determines when particular pixels included in the focus setting area are scanned. When those pixels are scanned, as may be suitably established by timing control included in system controller 12, an auto-focus area enabling signal is produced. This enabling signal is used as a gating signal, as will be hereinafter described.

System controller 12 desirably controls auto-focus area setting circuit 24 to establish two or more focus detection areas, for example, one focus detection area is a relatively small discrete area AE1 included within a larger discrete area AE2, as shown in FIG. 11A. Alternatively, the two discrete focus detection areas may be as illustrated in FIG. 11B, wherein the two areas AE'1 and AE'2 extend laterally across the image and are of substantially equal dimensions, with one area being disposed vertically above the other. It will be appreciated that other focus detection areas may be used and that, if desired, three, four or more focus detection areas may be provided.

Auto-focus area setting circuit 24 has an output thereof supplied to a selector 30 to which outputs from an auto-exposure area setting circuit 25, an automatic white balance area setting circuit 26 and a display area setting circuit 27 also are supplied. The selector 30 is controlled by system controller 12 through the serial input port 28 to couple one of the outputs supplied thereto to an output terminal 47. As a result of the selected area setting output signal, a camera viewfinder (not shown) may display a corresponding area outline. The user of the camera thus may observe the focus detection area which is used by the auto-focus detector 21 to produce the focus detection signal. Similarly, the exposure detection area produced by circuit 25, the white balance detection area produced by circuit 26 or the display area produced by circuit 27 may be observed. The video camera may be provided with suitable manual controls (not shown) for causing the system controller 12 to select a desired one of these detection areas for display.

The manner in which the luminance and chrominance components are separated from the digitized pixels produced by scanning the checkerboard pattern shown in FIG. 3 will now be described. As mentioned previously, the checkerboard pattern is scanned so that successive vertical pairs of pixel elements are read. This successive scanning of pixel pairs produces the signal shown in FIG. 5A when rows L1 and L2 are scanned, and produces the signal shown in FIG. 5B when rows L1 and L3 are scanned. As shown in FIG. 5A, when pixel element Cy in row L1 is scanned concurrently with pixel element G in row L2, the resultant digitized pixel corresponds to a sum signal (Cy+G). When the next adjacent pixel element Ye of row L1 is scanned concurrently with the next adjacent pixel element M of row L2, the sum signal (Ye+M) is produced. FIG. 5A represents the summed pixels which are produced as successive pixel pairs in rows L1 and L2 are scanned.

Similarly, when rows L1 and L3 are scanned concurrently, the scanning of pixel element Cy in row L1 with pixel element M in row L3 results in the sum signal (Cy+M). Likewise, when pixel element Ye in row L1 is scanned concurrently with pixel element G in row L3, the sum signal (Ye+G) is produced. FIG. 5B represents the successive summed signals produced when pixel pairs in rows L1 and L3 are scanned in succession.

Chrominance separator 33 operates to subtract successive summed signals produced by the row-by-row scanning of the checkerboard array shown in FIG. 3. In particular, when the summed signal shown in FIG. 5A is produced, the digital sample (Cy+G) is subtracted from the digital sample (Ye+M). It is to be appreciated that a yellow signal is formed by adding red and green signals (Ye=R+G), a magenta signal is formed by adding red and blue signals (M=R+B) and a cyan signal is produced by adding blue and green signals (Cy=B+G). Therefore, the subtracting operation carried out by chrominance separator 33 when the summed signals of FIG. 5A are produced may be expressed as follows: ##EQU1##

Similarly, the subtracting operation carried out by chrominance separator 33 when the summed signals represented by FIG. 5B are produced may be expressed as follows: ##EQU2## Thus, chrominance separator 33 separates the chrominance components C_(R) and C_(B) from the digitized video signal supplied to input terminal 31 from A/D converter 9.

Luminance separator 32 operates by summing two successive digitized samples produced by the A/D converter. For example, when the samples shown in FIG. 5A are produced, the luminance separator 32 operates to sum these samples as follows: ##EQU3## Likewise, the luminance separator 32 operates to sum every two samples of the signal represented by FIG. 5B, as follows: ##EQU4##

In addition to separating the luminance and chrominance components as discussed above, separator circuits 32 and 33 convert the sampling frequency at which the digitized luminance and chrominance, components are produced. For example, if the checkerboard pattern of FIG. 3 is comprised of 510 pixel elements in each row, the sampling frequency is converted from (8/3)f_(sc) to a re-clocked rate of 2f_(sc). Alternatively, if the checkerboard pattern is comprised of 760 pixel elements in a row, the sampling frequency is converted from 4f_(sc) to 2f_(sc). The thus separated, digitized luminance signal is supplied from luminance separator 32 to the auto-focus detector 21 and also to the auto-exposure detector 22 and the auto-white balance detector 23. The separated chrominance components produced by chrominance separator 33 are supplied to the automatic white balance detector 23.

Returning now to the auto-focus detector 21 of FIGS. 4A and 4B, it will be seen that the digitized luminance signal Y is supplied to digital filters which serve to separate intermediate and higher frequency components from the luminance signal. As mentioned above, when lens group F4 is disposed at its just-focused position, sharp transitions are provided in the luminance signal, resulting in relatively high amplitudes of intermediate and higher frequency components (for example, frequencies which exceed 1 MHz and, preferably, those which exceed 500 KHz). The focus condition of the lens may be determined by detecting the amplitude of these intermediate and higher frequency components. For simplification, the expression "higher frequency components" is used hereafter to refer to both the intermediate and higher frequencies.

The digital filters receiving the digitized luminance signal Y are comprised of a delay circuit 34, conventional filter processors or computers 35, 36 and 37 and a high pass filter circuit 38. Delay circuit 34 may be a conventional digital delay line, such as may be formed by cascaded CCD's. Each filter processor or computer 35, 36 or 37 is coupled to respective outputs, or taps, of delay circuit 34, and each filter processor or computer is comprised of a summation circuit supplied with individually weighted outputs that are tapped from the delay circuit 34. By establishing different weighting coefficients coupled to different delay line taps, each of the conventional filter processors exhibits different filtering characteristics. Examples of the individual filtering characteristics of filter processors 35, 36 and 37 are graphically illustrated at 35a, 36a, and 37a, respectively, on FIG. 6. FIG. 6 also graphically represents at 38a an example of the filtering characteristic of the high pass filter 38. It is seen that the cut-off frequencies of filters 35, 36, 37 and 38 increase successively, as shown in FIG. 6.

As mentioned previously, the amplitude, or level, of the higher frequency components included in luminance signal Y is at a maximum when lens group F4 is at its just-focused position, and this amplitude, or level, decreases as the lens moves from that position to increase the extent to which it is unfocused. Accordingly, a relationship may be established between the position of lens group F4 and the amplitude or level of the filtered signals derived from filter processors 35, 36 and 37 and high pass filter 38. This relationship is graphically illustrated in FIG. 7, and it is seen therefrom that the spectral characteristics 35', 36', 37' and 38' of filter processors 35, 36 and 37 and high-pass filter 38, respectively, tend to become "sharper", or more narrowly defined, from filter processor 35 to high pass filter 38. These different filter characteristics are used by auto-focus detector 21 to produce a focus detection signal which may be used to drive lens group F4 accurately and quickly to its just-focused position.

Selector circuits 39A, 39B, 39C and 39D, which may be comprised of suitable digital switches, are controlled by a selection signal produced by system controller 12 and supplied to the selector circuits 39A-39D through serial input port 28. In the illustrated example, each selector circuit includes three inputs a, b and c, any one of which may be selected to be connected to the output thereof. Inputs a, b and c of selector circuits 39A and 39B are coupled to the outputs of filter processors 35, 36 and 37, respectively. Inputs a, b and c of selector circuits 39C and 39D are coupled to the outputs of filter processors 36 and 37 and of high pass filter 38, respectively. Thus, depending upon the operation of the selector circuits 39A-39D, a particular filter characteristic may be selected to pass the higher frequency components of the digitized luminance signal Y.

Coring circuits 40A, 40B, 40C and 40D are coupled to selector circuits 39A, 39B, 39C and 39D, respectively. Each of the coring circuits 40A-40D is comprised of a threshold detector adapted to detect when the amplitude or level of the higher frequency component supplied thereto from the respective one of the selector circuits 39A-39D exceeds a minimum threshold level. That minimum threshold level is determined for the coring circuits 40A=14 40D by a suitable control signal supplied thereto from the system controller 12 through the serial input port 28. This minimum threshold level is approximately equal to the expected noise signal level included in luminance signal Y when the video camera images a scene with relatively little contrast changes therein.

The function of coring circuits 40A-40D will best be appreciated by referring to the waveform diagrams of FIGS. 8A and 8B. When the video camera images a scene having relatively low contrast, such as a scene that appears as a relatively simple pattern, the noise signal N which passes through the digital filters 35 may be erroneously detected as a higher frequency component of the luminance signal. As a result, such erroneous detection of the noise signal N may produce a focus detection signal which will be used by system controller 12 to perform an erroneous focus operation. However, by detecting only those signal levels which exceed a threshold above the expected noise signal level N, such as those signal levels which exceed threshold V₁ shown in FIG. 8A, erroneous detection of the noise signal level is substantially avoided. Coring circuits 40A-40D are supplied with threshold level V₁ and thereby detect only the signal levels of the higher frequency components which exceed the level V₁. Consequently, noise signals are rejected and substantially only true signal levels of the higher frequency components pass through the coring circuits, as shown in FIG. 8B.

Each of the coring circuits 40A-40D may be implemented by a subtractor which subtracts the threshold level V₁ from the signal supplied from the respective one of the digital filters. If the threshold level V₁ exceeds the signal level from which it is subtracted, the respective coring circuit produces a substantially zero output. Thus only those higher frequency components which exceed the threshold level pass through the coring circuits 40A-40D.

Preferably, threshold level V₁ is changed automatically as a function of the intensity of the total light level picked up by imaging device 2. Alternatively, this threshold level V₁ may be changed manually, as desired.

The outputs of coring circuits 40A, 40B, 40C and 40D are selectively supplied through gate circuits 41A, 41B, 41C and 41D and switch circuits 42A, 42B, 42C and 42D to peak detectors 43A, 43B, 43C and 43D, respectively. The gate circuits 41A-41D are supplied with gate enabling signals produced by auto-focus area setting circuit 24 to pass only those higher frequency components produced from the pixel elements of the checkerboard pattern that are disposed within the auto-focus area established by setting circuit 24. For example, gate circuits 41A and 41B may be enabled, or opened, to pass the higher frequency components produced when scanning within larger focus detection area AE2 shown in FIG. 11A, while gate circuits 41C and 41D are enabled to pass the higher frequency components produced from the pixel elements disposed within the smaller focus detection area AE1. As will be described, gate circuits 41A and 41B are used when the lens is relatively far from its just-focused position, and gate circuits 41C and 41D are used when the lens is proximate to its just-focused position.

Gate circuits 41A-41D are further controlled to respond to a gate inhibit signal generated by auto-focus area setting circuit 24 when the level of the luminance signal Y produced by imaging device 2 is too high. As mentioned above, if the scene being imaged by the video camera contains a portion with a very high brightness level, higher frequency components derived from the bright luminance signal ma exhibit an excessively high amplitude. However, this high amplitude is due to discrete overly bright areas and not to sharp transitions in the luminance signal that otherwise are present when the lens is at its just-focused position. Consequently, if this high amplitude in the higher frequency components of the luminance signal due to overly bright discrete areas is not blocked, errors will be made in the auto-focusing operation.

To prevent such errors, auto-focus area setting circuit 24 supplies a gate inhibit signal to gate circuits 41A-41D in the event that the luminance signal Y exceeds a maximum threshold. A luminance or brightness detector 46 is coupled to the luminance separator 32 to sense when the magnitude of the luminance signal exceeds a predetermined maximum threshold V₂. Hence, the gate circuits 41A-41D are disabled or inhibited for the time period that the luminance signal exceeds this maximum threshold V₂.

The function and advantages of the luminance detector 46 will be more specifically described with reference to the waveform diagrams shown in FIGS. 9A-9D. FIG. 9A represents the luminance component produced by imaging device 2 and, although analog signal levels are depicted, it will be appreciated that, in actuality, the signal levels are represented in digital form. FIG. 9A illustrates an example in which the level of the luminance signal exceeds the maximum threshold V₂ during the time T, as will occur when the imaging device images a scene having a very high brightness level, such as a scene that contains an object of high reflectivity, a bright light source, or the like. FIG. 9B represents the higher frequency components of the luminance signal of FIG. 9 that pass through one of the digital filters 35, 36, 37 and 38. It will be appreciated that the amplitude of the higher frequency components produced when the luminance signal Y exceeds maximum threshold V₂ is quite high and may provide false indications of the focus condition of the lens.

As shown in FIG. 9C, luminance detector 46 generates an inhibit or masking signal of a duration equal to the time T during which the luminance signal Y exceeds threshold V₂. This masking signal is supplied to auto-focus area setting circuit 24 which responds thereto to generate a gate inhibit signal by which

gate circuits 41A-41D are closed, or inhibited, during this interval; whereby the false higher amplitude components caused by the excessive brightness level in the luminance signal do not pass through the gate circuits. This selective operation of the gate circuits is illustrated in FIG. 9D, wherein the gate circuits are inhibited during interval T, that is, the interval during which the luminance signal Y exceeds the maximum threshold V₂. Consequently, only the higher frequency components having non-excessive signal levels that are received from the coring circuits 40A-40D are passed by the gate circuits 41A-41D.

Alternatively, luminance detector 46 may function to sense when the luminance signal Y exceeds maximum threshold V₂ and then subtract this luminance signal level from the luminance signal itself, thereby cancelling the excess brightness, whereupon, this "corrected" luminance signal may be supplied to the digital filters for obtaining the higher frequency components.

Peak detector circuits 43A, 43B, 43C and 43D are coupled to gate circuits 41A, 41B, 41C and 41D by switching circuits 42A, 42B, 42C and 42D, respectively. The switching circuits 42A-42D are selectively controlled by system controller 12 which applies suitable switch control signals thereto through serial input port 28.

Each peak detector circuit, although formed as a digital circuit, operates to detect the peak level (that is, the maximum level) in the signal supplied through the switching circuit connected thereto. By way of example, each switching circuit, such as, switching circuit 42A, couples the output of the respective gate circuit 41A to peak detector circuit 43A for the duration of a video line interval, whereafter the switching circuit 42A is opened and the peak detector circuit 43A is reset. In this mode of operation, the peak level of the higher frequency components included in the luminance signal Y during each video line interval is detected.

Alternatively, each switching circuit, for example, switching circuit 42A may be closed for an entire vertical field interval. In that event, the respective peak detector circuit 43A detects the peak (or maximum) level in the higher frequency components included in the luminance signal that is produced during a video field. At the end of a field interval, switching circuit 42A is opened and peak detector circuit 43A is reset in preparation to detect the peak level included in the next field interval.

Each switching circuit, such as switching circuit 42A, also is operable to bypass peak detector circuit 43A so as to pass all peaks included in the higher frequency components provided at the output of the respective coring circuit 40A and gate circuit 41A (rather than passing only the maximum peak level included in the higher frequency component).

Integrator circuits 45A, 45B, 45C and 45D are coupled to peak detector circuits 43A, 43B, 43C and 43D by switching circuits 44A, 44B, 44C and 44D, respectively. The switching circuits 44A-44D are controlled by system controller 12 which supplies a control signal thereto through serial input port 28. Each of the switching circuits 44A-44D selectively exhibits a first state in which the output of the respective one of the peak detector circuits 43A-43D is coupled to a respective one of the integrator circuits 45A-45D, and a second state in which the respective one of the integrator circuits 45A-45D is bypassed. Each of the integrator circuits 45A-45D is adapted to sum the peaks supplied by the respective one of the peak detector circuits 43A-43D connected thereto. By way of example, the peak (or maximum) level sensed by peak detector circuit 43A in each line of a field interval is summed by integrator circuit 45A to produce a focus detection signal whose magnitude is dependent upon (and, thus, is indicative of) the focus condition of the lens. Alternatively, integrator circuit 45A may be supplied with the peak, or maximum, level sensed by peak detector circuit 43A in a vertical field interval. The integrator circuit 45A then sums the detected peaks that are sensed in a predetermined number of field intervals, such as the peaks that are detected in four successive field intervals. This too provides a focus detection signal whose magnitude is dependent upon the level of the higher frequency components included in the luminance signal which, in turn, is a function of the focus condition of the lens.

In the mode of operation of switching circuit 42A for bypassing peak detector circuit 43A, there are supplied to integrator circuit 45A those higher frequency components which exceed threshold V₁ established by coring circuit 40A. In that case, integrator circuit 45A sums a number of peak levels included in a field interval, provided those peak levels exceed the threshold V₁.

The foregoing alternative operating modes of peak detector circuit 43A and integrator circuit 45A will best be appreciated by referring to FIG. 10 which is a part schematic, part block diagram of these circuits shown interconnected by switching circuits 42A and 44A which are illustrated as two-position switches. In its first position shown in full lines, switching circuit 42A couples the output of gate 41A to peak detector circuit 43A, and in its second position shown in dotted lines the switching circuit 42A bypasses the peak detector circuit 43A. Similarly, switching circuit 44A is changeable between a first position shown in full lines for coupling the output of peak detector circuit 43A to integrator circuit 45A, and a second position shown in dotted lines to bypass the integrator circuit 45A. Switching circuits 42A and 44A are operable by switch control signals supplied thereto through the serial input port 28 from the system controller 12 so as to establish the following modes:

(a) Both switching circuits 42A and 44A assume their first positions, as shown in full lines, and peak detector circuit 43A is suitably reset at the end of each line interval, whereupon peak detector circuit 43A detects the peak (or maximum) level in the higher frequency components included in the luminance signal during each video line interval. Integrator circuit 45A sums the peak level detected in each line interval over a field duration.

(b) Switching circuit 42A assumes its first or full-line position and switching circuit 44A assumes its second position shown in dotted lines, whereupon peak detector circuit 43A, which is reset at the end of each field interval, senses the peak level in a field interval, and integrator circuit 45A is bypassed. Hence, the detected peak level is not summed (or integrated) with other peak levels. It will be appreciated that the thus detected peak level in a field interval is an indication of the focus condition of the lens.

(c) Switching circuit 42A assumes its second or dotted-line position while switching circuit 44A assumes its first or full-line position, whereupon all signal levels, including peaks, of the higher frequency components included in the luminance signal Y which exceed the threshold level V₁ established by coring circuit 40A are integrated over a field interval.

(d) Both switching circuits 42A and 44A assume their first or full-line positions, and peak detector circuit 43A is reset at the end of each field interval. In this mode, the peak detector circuit 43A detects the peak (or maximum) signal level in the higher frequency components over a video field interval, while the integrator circuit 45A is made to sum the detected peak levels for a predetermined number of fields, for example, the peak level in each of four successive fields, to produce the focus detection signal. In this mode of operation, the focus detection signal varies relatively slowly as compared to the other operating modes discussed above.

Although only the switching circuits 42A and 44A and their association with the peak detector circuit 43A and integrator circuit 45A have been shown in FIG. 10 and described specifically above, it will be understood that the switching circuits 42B, 42C and 42D and 44B, 44C and 44D are similarly associated with the peak detector circuits 43B, 43C and 43D and integrator circuits 45B, 45C and 45D, respectively.

The signals provided at the outputs of integrator circuits 45A-45D comprise focus detection signals having magnitudes to which the focus condition of lens group F4 is related. Such focus detection signals are supplied by way of serial output port 29 to the system controller 12, which, on the basis thereof, produces a focus control signal supplied by way of driver 13 to focus motor 3 in FIG. 1.

The manner in which a focusing operation is carried out will now be described. Initially, let it be assumed that lens group F4 is at an out-of-focus position, for example, at a position indicated at i in FIG. 7. System controller 12 initially controls selector circuits 39A and 39B so as to select the filtering characteristics exhibited by filter processor or computer 35, and thereby derive the higher frequency components in the luminance signal. The selected higher frequency components are applied to gates 41A and 41B through coring circuits 40A and 40B which provide noise rejection in the manner discussed above and shown in FIGS. 8A and 8B. Hence, the coring circuits 40A and 40B detect signal levels of the higher frequency components which exceed the threshold V₁ and supply the detected signals to gates 41A and 41B, respectively.

In this initial condition, auto-focus area setting circuit 24 is controlled by system controller 12 to generate a gate enable signal corresponding to the auto-focus area AE₂, shown in FIG. 11A. Accordingly, the detected signal produced by coring circuit 40A is gated to peak detector circuit 43A when the pixel elements of imaging device 2 which are included within auto-focus area AE₂ are being scanned. Let it be assumed that peak detector circuit 43A detects the peak (or maximum) level of the detected signal during each video line interval. The detected peaks in those line intervals which are disposed within auto-focus area AE₂ are summed in integrator circuit 45A to produce a focus detection signal.

Coring circuit 40B, gate 41B, peak detector circuit 43B and integrator circuit 45B operate in the same way as just described. Hence, integrator circuit 45B produces a focus detection signal similar to that produced by integrator circuit 45A. Both of these focus detection signals are supplied serially to system controller 12. At this time, the signals that may be produced by integrator circuits 45C and 45D are ignored and, preferably, are not supplied to or utilized by the system controller.

In the described arrangement of the optical detector 11, the focus detection signal produced by one of a pair of integrator circuits, such as the focus detection signal produced by integrator circuit 45A, is used as a measure of the focus condition of the lens. The other focus detection signal, for example, the focus detection signal produced by integrator circuit 45B, is used by the system controller 12 to determine when a filter processor having sharper filtering characteristics should be selected. For example, as the lens group F4 is driven toward its just-focused position, the magnitude of the focus detection signal derived by integrator circuit 45B from filter processor 35 changes at a rate which diminishes as the lens approaches its just-focused position. This diminished rate of change is attributed to the decrease in slope of the spectral characteristic 35' of filter processor 35, as illustrated in FIG. 7.

When the lens group F4 reaches position ii (shown in FIG. 7), the sensed decrease in the rate of change of the focus detection signal derived by the integrator circuit 45B from the output of filter processor 35, and which is represented by the slope of curve 35', causes the system controller 12 to operate selector circuits 39A and 39B for selecting filter processor 36 (having a narrower filter characteristic 36' on FIG. 7) for passing the higher frequency components included in the luminance signal Y. Coring circuits 40A and 40B now detect the signal levels of the higher frequency components supplied thereto from filter processor 36, and produce detected signals when these signal levels exceed threshold V₁. Thus, even though different filtering characteristics are selected for deriving the higher frequency components of the luminance signal, noise rejection nevertheless is carried out by the coring circuits 40A and 40B. Gates 41A and 41B continue to gate the detected signals during the interval corresponding to auto-focus area AE₂, and peak detector circuits 43A and 43B detect the peak level in each video line. Integrator circuits 45A and 45B sum the peak signal in each line over a field interval to produce the focus detection signals. As before, the focus detection signal provided by integrator circuit 45A is used as a measure of the focus condition of the lens, and the focus detection signal provided by integrator circuit 45 is used by the system controller to determine when the filter characteristics should be changed over to a narrower characteristic. Once again, a determination to change over to a narrower filter characteristic is made when the rate of change of the focus detection signal decreases.

Let it now be assumed that, when the focus lens reaches position iii (shown in FIG. 7), the focus detection signal derived from filter processor 36 increases at a slower rate, as represented by the slope of curve 36' at 36iii. The system controller 12 responds thereto by selecting filter processor 37, having the spectral characteristic curve 37' illustrated in FIG. 7, to derive the higher frequency components from the luminance signal Y. More specifically, selector circuits 39A and 39B now are changed over to couple the output of filter processor 37 to the respective coring, gate, peak detector and integrator circuits. These circuits operate in the manner described above to produce the focus detection signals.

Selector circuits 39A and 39B, coring circuits 40A and 40B, gate circuits 41A and 41B, peak detector circuits 43A and 43B and integrator circuits 45A and 45B are used to produce focus detection signals during scanning of the pixel elements within auto-focus area AE₂. Selector circuits 39C and 39D, coring circuits 40C and 40D, gate circuits 41C and 41D, peak detector circuits 43C and 43D and integrator circuits 45C and 45D are used to produce the focus detection signals during scanning of the pixel elements within auto-focus area AE₁. Auto-focus area setting circuit 24 supplies gate enable signals to at least gate circuits 41A and 41B to define auto-focus area AE₂ for a predetermined time. It is appreciated from the foregoing discussion that during this time, the focusing lens is driven toward its just-focused position. At the expiration of this predetermined time, the auto-focus area setting circuit supplies gate enable signals to at least gate circuits 41C and 41D to define auto-focus area AE₁ (FIG. 11A). When focus detection signals are derived from the higher frequency components of the luminance signal produced in response to scanning of the pixel elements within auto-focus area AE₁, the outputs of integrator circuits 45A and 45B may be ignored and, if desired, need not even be supplied to system controller 12 through serial output port 29. Alternatively, if these focus detection signals are supplied to the system controller 12, they are not used thereby.

Selector circuits 39C and 39D are each connected to filter processors 36 and 37 and also to high pass filter 38 and, under the control of system controller 12, they select the spectral characteristics of any one of these filtering circuits in the same manner as selector circuits 39A and 39B select the spectral characteristic of filter processor 35, 36 or 37. For example, let it be assumed that the predetermined time interval, in which the auto-focus area AE₂ is defined or being scanned, expires shortly after the focusing lens group F4 reaches position iii on FIG. 7, at which point filter processor 37 was selected by selector circuits 39A and 39B to derive the higher frequency components of the luminance signal. System controller 12 now selects selectors 39C and 39D, together with the circuits connected in cascade therewith, to produce the focus detection signals. Selectors 39C and 39D are controlled to select filter processor 37 for deriving the higher frequency components. Coring circuits 40C and 40D detect signal levels of these higher frequency components which exceed the threshold V₁, and gate circuits 41C and 41D gate the detected signals produced by the coring circuits to peak detector circuits 43C and 43D during scanning of pixel elements within auto-focus area AE₁. Peak detector circuits 43C and 43D detect the peak (or maximum) level in the gated, detected signal during each video line interval, and integrator circuits 45C and 45D sum the detected peaks in the line intervals disposed within auto-focus area AE₁. Hence, integrator circuits 45C and 45D produce focus detection signals in the same manner as discussed above in conjunction with integrator circuits 45A and 45B.

As the focusing lens group F4 continues to advance toward its just-focused position, system controller 12 changes over selector circuits 39C and 39D when the lens arrives at, for example, position iv (shown in FIG. 7). More specifically, the system controller 12 senses that the rate of change of the focus detection signal produced by integrator circuit 45D has decreased and, in response thereto, changes over selector circuits 39C and 39D to select the narrowest spectral characteristic 38' which is exhibited by high pass filter 38.

The foregoing operation is repeated and, when the focus detection signal produced by integrator circuit 45C attains a predetermined level, system controller 12 determines that the focusing lens has reached its just-focused position. Accordingly, focus drive motor 3 (FIG. 1) is stopped.

Alternatively, system controller 12 may sense when the focus detection signal produced by integrator circuit 45C reaches its peak level (rather than reaching a predetermined level), whereupon the focus drive motor 3 is stopped.

As mentioned above, in order to prevent spurious signal levels which may be present erroneously in the detected signal produced by the coring circuits due to very high brightness levels in the luminance signal Y, gate circuits 41A-41D are disabled during those periods that such excessive brightness levels are present. More specifically, luminance detector 46 controls auto-focus area setting circuit 24 to produce a gate inhibit signal when excessive brightness levels are sensed, as discussed above in conjunction with FIGS. 9A-9D. Thus, integrator circuits 45A-45D function to integrate the detected peaks produced by peak detector circuits 43A-43D, except during those intervals when the luminance signal exceeds threshold V₂.

In the operation just described, the higher frequency components derived from selector circuits 39A and 39B are used to produce the focus detection signals for a predetermined time, for example, during scanning of pixel elements in the auto-focus detection area AE₂, whereafter the higher frequency components derived by selector circuits 39C and 39D are used to produce the focus detection signals during scanning of pixel elements within auto-focus detection area AE₁. Alternatively, selector circuits 39A and 39B may be selected to derive the higher frequency components within auto-focus detection area AE₂ until the rate of change of the focus detection signal produced by integrator circuit 45B is too slow (even after filter processors with narrower spectral characteristics have been selected by selector circuits 39A and 39B). Thereafter, selector circuits 39C and 39D are selected for deriving the higher frequency components from which the focus detection signals while scanning pixel elements within auto-focus detection area AE₁ are produced. Other techniques may be used to change over from one auto-focus area to another and, accordingly, to change over from one set of selector circuits to another.

In the arrangement described above, selector circuits 39A and 39B are controlled to select the same filtering characteristics provided by the digital filter and, similarly, selector circuits 39C and 39D also are controlled to select the same filtering characteristics. Alternatively, select or circuit 39A may be controlled to select a filter characteristic having broader (or wider) spectral characteristics than the filter characteristic selected by selector circuit 39B. Likewise, selector circuit 39C may be controlled to select a frequency characteristic having broader spectral characteristics than the filter characteristic selected by selector circuit 39D. For example, when the focusing lens is at position i (shown in FIG. 7), selector circuit 39A may select the filtering characteristics exhibited by filter processor 35 and represented by the curve 35', whereas selector circuit 39B selects the filtering characteristic exhibited by filter processor 36 and represented by the curve 36'. Hence, the focus detection signal produced by integrator circuit 45A is derived from filter processor 35 and the focus detection signal produced by integrator circuit 45B is derived from filter processor 36. As before, when system controller 12 senses that the rate of change of the focus detection signal produced by integrator circuit 45B from the higher frequency components passed by filter processor 36 is too slow, control signals are supplied from the system controller 12 through input port 28 to selector circuits 39A and 39B so that selector circuit 39A now selects the filtering characteristics exhibited by filter processor 36 and selector circuit 39B now selects the filtering characteristics exhibited by filter processor 37. The foregoing operation then is repeated until, once again the system controller 12 senses that the rate of change of the focus detection signal produced by integrator circuit 45B is too slow. At that time, selector circuit 39A is changed over to select the filtering characteristics exhibited by filter processor 37.

Similarly, when the focus detection signals are derived from the higher frequency components in the luminance signal produced during scanning of pixel elements in the focus detection area AE₁, selector circuit 39C may be controlled to select the filtering characteristics exhibited by filter processor 36 while selector circuit 39B is controlled to select the filtering characteristics exhibited by filter processor 37. As before, when system controller 12 senses that the rate of change of the focus detection signal produced by integrator circuit 45D is too slow, selector circuit 39C may be changed over to select the filtering characteristics exhibited by filter processor 37 and selector circuit 39D is changed over to select the filtering characteristics exhibited by high pass filter 38. Further changeovers occur in the selection of the filtering characteristics as the focusing lens approaches its just-focused position, as described above.

In accordance with yet another alternative mode of operation, the selector circuits 39A-39D may be controlled so that each selects a respective predetermined filtering characteristic. For example, selector circuit 39A may select the filtering characteristic exhibited by filter processor 35, selector circuit 39B may select the filtering characteristic exhibited by filter processor 36, selector circuit 39C may select the filtering characteristic exhibited by filter processor 37 and selector circuit 39D may select the filtering characteristic exhibited by high pass filter 38. Assuming that the focusing lens group F4 is relatively distant from its just-focused position, the auto-focus detection area AE₂ is used and gates 41A and 41B are enabled at intervals corresponding to this auto-focus area. In such case, the focus detection signal from integrator circuit 45A is used by system controller 12 to determine the focus condition of the lens. The system controller 12 also senses the rate of change of this focus detection signal and, when that rate decreases, selector circuit 39B is actuated to select the higher frequency components derived from the luminance signal by filter processor 36. Now, the focus detection signal produced by integrator circuit 45B is used as an indication of the focus condition of the lens.

System controller 12 controls selector circuits 39C and 39D in a manner similar to that just described when the focus detection signal is derived from scanning of pixel elements in the auto-focus detection area AE₁.

In yet another operating mode, a pair of focus detection signals is used as a measure of the focus condition of the lens. For example, the focus detection signals produced by integrator circuits 45A and 45B ar used in combination to sense the focus condition during scanning of pixel elements within auto-focus detection area AE₂, and the focus detection signals produced by integrator circuits 45C and 45D are used as a measure of the focus condition during scanning of pixel elements within the auto-focus detection area AE₁. By using a pair of focus detection signals, erroneous indications attributable to artifacts or interference in one of the focus detection signals are avoided. For the purposes of this example, it will be assumed that the focusing lens is distant from its just-focused position, and that selector circuit 39A is controlled to select the filtering characteristics exhibited by filter processor 35 while selector circuit 39B is controlled to select the filtering characteristics exhibited by filter processor 36. If an impulse or other interference is present on the focus detection signal produced by integrator circuit 45A, that interference could be erroneously interpreted by the system controller 12 as indicating that the focusing lens is much closer to its just-focused position than it is in fact. Responding to such false interpretation, the system controller may change over selector circuit 39A prematurely to select the filtering characteristics exhibited by filter processor 36 or even filter processor 37. Because of the narrow spectral characteristics exhibited by the prematurely selected filter processor, the amplitude of the focus detection signal derived therefrom may be too low for the system controller to determine the actual focus condition of the lens. However, since it is unlikely that similar interference Will be also superimposed on the focus detection signal produced by integrator circuit 45B, the system controller 12 may compare the focus detection signals produced by integrator circuits 45A and 45B to conclude that the higher magnitude of one is not matched by the other and, thus, is artificial. Hence, the system controller 12 may discriminate artifacts, noise or interference superimposed onto a focus detection signal, and thereby avoid false indications of the lens focus condition.

From the foregoing, it is to be appreciated that, by changing the filtering characteristics used to derive higher frequency components from the luminance signal produced by the video camera imaging device as the focusing lens is driven toward its just-focused position, the lens may be focused quickly and accurately. Optimum filtering characteristics are selected as the lens approaches its just-focused position. Moreover, coring circuits 40A-40D prevent erroneous focusing operations that may be attributed to noise signals which pass through the filter processors when a low contrast scene is being imaged. Also, focusing errors which otherwise may be due to the imaging of very bright areas are avoided by the brightness detector 46 cooperating with the auto focus area setting circuit 24 for causing the gate circuits 41A-41D to reject the higher frequency components having high amplitudes caused by such bright areas, whereby such higher frequency components having high amplitudes do not influence the focus detection signals produced by integrators 45A-45D.

AUTO-EXPOSURE DETECTOR 22

In the video camera to which the invention is desirably applied, auto-exposure control is effected by suitably opening and closing the iris 18 and by varying the gain of the AGC circuit 8 (FIG. 1) so that the level of the luminance signal from an exposure detection area of the CCD imaging device 2 may be maintained at a predetermined value.

Since the brightness level of the background of a scene which is back-lighted becomes very high, the overall luminance signal level is substantially elevated and, if the auto-exposure control is effected with reference to such overall luminance signal level, the aperture size of the iris 18 is decreased and a relatively low gain is established for the AGC circuit. By reason of the foregoing, the output of the video camera would cause objects in the foreground of the corresponding displayed image to appear dark and inconspicuous. On the other hand, when the scene is subjected to excessive lighting from in front (hereinafter referred to as excessive front-lighting), the brightness level of the background is low relative to the brightness of objects in the foreground and the overall luminance signal level is decreased so that, in response thereto, the iris is opened and the gain of the AGC circuit is increased with the result that the brightly illuminated objects in the foreground of the displayed image would be saturated.

Generally, in the illustrated video camera the above problems encountered in effecting exposure control are avoided by dividing the image of a scene in the field of view of the video camera into a plurality of exposure detection areas whose positions and relative dimensions are controlled in accordance with lighting conditions, such as, normal front-lighting, back-lighting or excessive front lighting, so that at all times exposure control can be effected with reference to a luminance signal level detected for an exposure detection area which contains foreground objects for maintaining a predetermined luminance signal level in respect to such foreground objects.

Furthermore, if exposure control is effected with reference to an average of the luminance signal levels detected for a selected exposure detection area the detected luminance signal level will be undesirably low. For example, in the case of a standard signal, such as, a color bar, the average luminance signal level is approximately 30 percent of the peak level. As a result, if exposure control of a video camera is carried out with a control signal obtained by average-detecting the luminance signal, the entire display screen is liable to appear dark. Therefore, it is desirable to effect exposure control with reference to a detected luminance signal value that is closer to the peak value of the luminance signal levels in the exposure detection area than it is to the average of the luminance signal levels in such detection area.

Therefore, in the auto-exposure detector 22 shown in FIG. 4B, the luminance component Y from luminance separator 32 is supplied to a so-called "knee" circuit 51 and to one input of a comparator 52. A comparison level signal, for example, of the level V₃ (FIG. 13B), is supplied to another input of the comparator 52 through the serial input port 28 from the system controller 12.

An output of the filter processor 37 is supplied to peak detector circuits 54A and 54B through gate circuits 53A and 53B, respectively. The output of the filter processor 37 which is supplied to gate circuits 53A and 53B is a luminance signal from which a high frequency range noise component has been removed by a low-pass filter. Thus, the noise component is removed from the luminance component supplied through gate circuits 53A and 53B to peak detector circuits 54A and 54B for accurate detection of the peaks therein. Since a digital high-pass filter based on a digital averaging low-pass filter is composed of the delay circuit 34 and the filter processor 37, both a high-pass filter output, for example, to be applied to the selector circuits 39A-39D, and a low-pass filter output, for example, to be applied to the gate circuits 53A and 53B, can be readily extracted from the filter processor 37.

The gate circuits 53A and 53B are supplied with gate enabling signals supplied from the auto-exposure detection area setting circuit 25 under the control of the system controller 12 for selectively opening and closing the gate circuits 53A and 53B and thereby defining a plurality of exposure detection areas, for example, as at AE₁ and AE₂ on FIG. 11A, or as at AE'₁ and AE'₂ on FIG. 11B.

As is shown on FIG. 12, the knee circuit 51 has a non-linear characteristic, that is, a characteristic comprised of two relatively straight portions with different slopes joined at the knee k₁. The output of the knee circuit 51 is supplied through gate circuits 55A and 55B to integrator circuits 56A and 56B, respectively. The gate circuits 55A and 55B are also supplied with a gate enabling signal from the auto-exposure detection area setting circuit 25 for defining the exposure detection areas AE₁ and AE₂ or AE'₁ and AE'₂. It will be apparent that the integrator circuits 56A and 56B detect averages of the luminance signal levels in the digital luminance component Y supplied to the integrator circuits, 56A and 56B when the respective gate circuits 55A and 55B are open or enabled. In the absence of the knee circuit 51, if an average of the luminance signal level is detected by the integrator circuit 56A or 56B during the interval in which the respective gate circuit 55A or 55B is open, the averaged output may be increased by a high brightness occurring at even a relatively small area within the respective exposure detection area, with the result that the auto-exposure control causes the entire picture output by the video camera to have an undesirably dark appearance. However, by the presence of the knee circuit 51, the gain of the high brightness portion is decreased so that the averaged output is not unduly influenced thereby. A characteristic setting signal is supplied from the system controller 12 through the serial input port 28 to the knee circuit 51 for varying the turning point or knee k₁ of the characteristic curve thereof, for example, between the positions shown on FIGS. 15A and 15B, respectively.

The comparator 52 detects those samples of the digital luminance component Y from the luminance separator 32 which have levels above the comparison level V₃ supplied to the comparator 52 from the system controller 12 through the serial input port 28. The resulting comparison output of the comparator 52 is supplied through gate circuits 57A and 57B to distribution detectors 58A and 58B. The distribution detectors 58A and 58B count the numbers of samples of the digital luminance signal having levels above the predetermined brightness level V₃ during intervals when the gate circuits 57A and 57B are respectively enabled or open. The gate circuits 57A and 57B are provided with the gate enabling signals from the auto-exposure detection area setting circuit 25 for defining the exposure detection areas AE₁ and AE₂ or AE'₁ and AE'₂.

The distribution of the luminance signal levels over the scene in the field of view of the video camera can be determined from the outputs of the distribution detectors 58A and 58B. For example, when a picture or scene in the field of view of the video camera is back-lighted, as illustrated in FIG. 13A, high level luminance signal portions are distributed mostly over the background or peripheral portions of the picture, while relatively low level luminance signal portions, for example, corresponding to a back-lighted object in the foreground of the picture, are distributed mostly in the central portion thereof, as shown in FIG. 13B. Such distribution of the high-level luminance signal portions can be determined from the count value constituting the output H1 of the distribution detector 58A and indicating the number of samples having a luminance level above the predetermined level V₃, for example, in the exposure detection area AE₁, and from the count value constituting the output H2 of the distribution detector 58B and which indicates the number of samples having luminance signal levels above the predetermined level V3, for example, in the exposure detection area AE₂. Such count values or outputs H1 and H2 from the distribution detectors 58A and 58B are supplied to the system controller 12 through the serial output port 29.

Outputs P₁ and P₂ representing luminance signal peak values for the selected exposure detection areas, such as the areas AE₁ and AE₂, are obtained from the peak detectors 54A and 54B, respectively, and are supplied through the serial output port 29 to the system controller 12. Similarly, outputs IN₁ and IN₂ representing the integrated values or averages of the luminance signal levels in the selected exposure detection areas, such as the areas AE₁ and AE₂, respectively, are obtained from the integrator circuits 56A and 56B, respectively, and are output through the port 29 to the system controller 12.

As earlier noted, if auto-exposure control is effected with reference only to an average of the detected luminance signal levels, for example, as obtained at the outputs IN₁, and IN₂ of integrator circuits 56A and 56B, the resulting exposure control is defective as the average detected level is undesirably low. Since desirable auto-exposure control is performed with reference to a detected luminance signal level which is nearer to the detected peak luminance signal level than to the average of the detected luminance signal levels, auto-exposure control is desirably effected with reference to a detected luminance signal level which is obtained by appropriately mixing average and peak values as derived from the integrator circuits 56A and 56B and from the peak detectors 54A and 54B, respectively.

In order to effect such mixing, and as shown in the functional block diagram of FIG. 14, the peak values P₁ and P₂ output by the peak detector circuits 54A and 54B and the average or integrated values IN₁ and IN₂ output by the integrator circuits 56A and 56B are applied to multipliers 71A, 71B, 72A and 72B, respectively, to be multiplied therein by suitable coefficients. The resulting modified or multiplied outputs of the multipliers 71A and 72A are added to each other in an adder 73A, and the outputs of the multipliers 71B and 72B are similarly added to each other in an adder 73B. It will be appreciated that, by suitably selecting the coefficients employed in the multipliers 71A and 71B as compared with the coefficients employed in the multipliers 72A and 72B, the outputs of the adders 73A and 73B can be provided with detection characteristics that are near to peak detection of the luminance signal levels in the respective exposure detection areas AE₁ and AE₂ or AE'₁ and AE'₂. Obviously, by changing the coefficients employed in the multipliers 71A and 71B and in the multipliers 72A and 72B, the detection levels can be varied. Since these computations are readily carried out by software, the detection levels can be easily changed.

On the basis of the value obtained by appropriately weighting and adding the detected value of the luminance signal level in the exposure detection area AE₁ or AE'₁ and the detected value of the luminance signal level in the exposure detection area AE₂ or AE'₂, opening and closing of the iris 18 and changing of the gain of the AGC circuit 8 are suitably controlled.

More particularly, as shown on FIG. 14, the outputs of the adders 73A and 73B are supplied to multipliers 74A and 74B, respectively, and the outputs of the multipliers 74A and 74B are added to each other in an adder 75, with the added output of the latter being applied to the system controller I2 as the exposure detection signal for determining the opening and closing of the iris 18 and the gain of the AGC circuit 8. It is to be noted that the multipliers 74A and 74B are provided for suitably weighting the detection value of the luminance signal level in one of the exposure detection areas, for example, the central area AE₁, and the detection value of the luminance signal level in the other exposure detection area, for example, the peripheral area AE₂. The weighting of the detection values in the multipliers 74A and 74B, respectively, is desirably dependent on the lighting conditions, for example, on whether the scene in the field of view of the video camera is being subjected to normal forward-lighting, back-lighting or excessive forward-lighting, as determined from the outputs H₁ and H₂ of the distribution detectors 58A and 58B, respectively.

In respect to the foregoing, it will be noted that normal forward-lighting of a scene in the field of view of the video camera results in substantially uniform brightness throughout the picture, that is, distribution of the luminance signal level in the exposure detection area AE₁ or AE'₁ which usually contains the principal object or objects, is substantially equal to that in the exposure detection area AE₂ or AE'₂ which contains the background of the picture. In other words, in the case of normal forward-lighting of the scene, the difference between the output H₁ of the distribution detector 58A and the output H₂ of the distribution detector 58B is not large.

On the other hand, when the scene in the field of view of the video camera is back-lighted, the background of the picture is extremely bright relative to objects in the foreground so that luminance signal levels above the predetermined value V₃ are mostly distributed in the peripheral exposure detection area AE₂ or the upper exposure detection area AE'₂. In other words, in the case of back-lighting, the output H₂ from the distribution detector 58B is large relative to the output H₁ from the distribution detector 58A.

Moreover, in the case of excessive forward-lighting of the scene in the field of view of the video camera, the background is dark relative to the very brightly lighted objects in the foreground. Therefore, in the case of excessive forward-lighting, luminance signal levels above the predetermined value V₃ are mostly distributed in the exposure detection area AE₁ or AE'₁ where the rightly illuminated objects are located, with the result that the output H₁ of the distribution detector 58A is then large relative to the output H₂ of the distribution detector 58B.

As shown in FIG. 14, the outputs H₁ and H₂ of the distribution detectors 58A and 58B are supplied to a brightness distribution state determining device 76 which, on the basis of a comparison of the outputs H₁ and H₂ relative to each other, determines whether the scene in the field of view of the video camera is being subjected to normal forward-lighting, back-lighting or excessive forward-lighting and, in response to such determination, provides corresponding outputs to the multipliers 74A and 74B for controlling or varying the coefficients employed therein.

More specifically, when the output H₂ is substantially greater than the output H₁ and the device 76 determines therefrom that the scene in the field of view of the video camera is subjected to back-lighting the coefficient of the multiplier means 74A for weighting the detection luminance signal level of the portion of the picture containing the principal objects is set to a large value, while the coefficient of the multiplier 74B for weighting the detection luminance signal level of the background portion is set to a relatively small value. On the other hand, when the output H₁ is substantially larger than the output H₂ and the device 76 determines therefrom that the scene in the field of view of the video camera is subjected to excessive forward-lighting, the coefficient of multiplier 74A for weighting the brightness of the object portion is set to a relatively small value and the coefficient of the multiplier 74B for weighting the brightness of the background portion is set to a relatively large value. As a result of the foregoing, the auto-exposure control approximates center-emphasized photometry, and optimum auto-exposure control is achieved even when the scene is subjected to back-lighting or excessive forward-lighting.

Further, the brightness distribution state determining device 76 is effective, in response to the outputs H₁ and H₂ from the distribution detectors 58A and 58B, to determine the positions and dimensions of the exposure detection areas, that is, to determine whether the exposure detection areas AE₁ and AE₂ (FIG. 11A) or the exposure detection areas AE'₁ and AE'₂ (FIG. 11B), are to be employed.

More specifically, in the case of normal forward lighting, that is, when the difference, if any, between the outputs H₁ and H₂ is not large, the distribution state determining device 76 provides an output at 76a to the system controller 12 by which the latter selects the arrangement of the exposure detection areas shown in FIG. 11B, that is, the arrangement in which the image projected on the CCD imaging device 8 is divided into an exposure detection area AE'₁ extending laterally across the lower portion of the entire image so as to contain the principal foreground objects, and an exposure detection area AE'₂ disposed vertically above the portion AE'₁ and also extending across the entire width of the image for containing the background. When the exposure detection areas AE'₁ and AE'₂ are selected in response to the normal forward-lighting of the scene, no variation occurs in the brightness as a principal object moves laterally, or in response to panning of the video camera, by reason of the fact that the principal objects remain within the exposure detection area AE'₁.

This may be contrasted with the arrangement of FIG. 11A in which, in response to panning of the camera or lateral movement of the object, the object would move laterally out of the centrally located exposure detection area AE₁.

When the scene in the field of view of the video camera is subjected to back-lighting or excessive forward-lighting, the resulting relatively large difference between the outputs H₁ and H₂ causes the distribution state determining device 76 to provide its output 76a with a level indicating to the system controller 12 that back-lighting or excessive front-lighting has occurred, whereupon the system controller 12 selects the exposure detection areas AE₁ and AE₂ shown on FIG. 11A. Thus, as previously noted, in the back-lighted and excessive forward-lighted conditions, the exposure detection area AE₁ which would contain a principal object is located at the center, whereas the exposure detection area AE₂ constitutes the peripheral portion and would contain the background. Apart from the different locations of the exposure detection areas in FIGS. 11A and 11B, it will be apparent that the exposure detection area AE₁ surrounded by the exposure detection AE₂ is of a substantially smaller size than the latter, whereas the exposure detection area AE'₁ is of a size that is at least as large as, or even slightly larger than the exposure detection area AE'₂. Due to the relatively smaller size of the centrally located exposure detection area AE₁, the exposure control effected with the exposure detection areas AE₁ and AE₂ of FIG. 11A approximates the desired center-emphasized photometry.

The output of the brightness distribution state determining device 76 may also be used by the system controller 12 for controlling the turning point or knee k₁ of the knee circuit 51. More particularly, when the device 76 determines from the outputs H₁ and H₂ that the scene in the field of view of the video camera is back-lighted, the turning point k₁ is shifted downwardly, for example, to the position shown on FIG. 15A. By reason of such downward shifting of the characteristic curve of the knee circuit 51, the luminance detection signal corresponding to the high brightness of the background due to the back-lighting will be subjected to a reduced gain in the knee circuit 51 so that the resulting exposure control will ensure that objects in the foreground do not appear dark in the back-lighted condition. On the other hand, when the distribution state detecting device 76 senses from the outputs H₁ and H₂ that the scene in the field of view of the video camera is subjected to excessive forward-lighting, the turning point k₁ of the knee circuit 51 is raised, for example, to the position shown on FIG. 15B. Thus, in the excessive forward-lighting condition, the gain of the knee circuit 51 is increased in respect to the luminance detection signal corresponding to the objects of high brightness appearing in the foreground of the excessively forward-lighted scene. By reason of the corresponding increase in the exposure control signal the opening of the iris 18 and the gain of the AGC circuit 8 are reduced for ensuring that the principal objects in the foreground of the picture do not saturate when subjected to excessive forward-lighting.

It will be apparent from the foregoing that, in effect, the gain of the entire exposure control system is set by the brightness distribution state determining device 76, that is, by changing the position of the knee k₁ of the knee device 51 as described above, and, as a result thereof, undesirable darkness of a principal object in the foreground of a back-lighted scene, and saturation of principal objects in the foreground of an excessively forward-lighted scene, are avoided.

It will be further appreciated that not all of the controls responsive to the back-lighting and excessive forward-lighting conditions are required. In other words, the problems associated with the back-lighting and excessive forward-lighting can be substantially avoided by employing one or more of the controls described above.

AUTOMATIC WHITE BALANCE DETECTOR 23

White balance control is generally carried out by controlling the levels of red (R), green (G) and blue (B) color signals so as to maintain a predetermined ratio thereof when viewing a white object or area. In the illustrated video camera according to an embodiment of the present invention, it is also possible to perform either fully automatic white balance control or so-called "one-push" auto white balance control. The fully automatic white balance control performs such control with reference to the integrated value of the entire picture which, for the purposes of the control, is assumed to be white. On the other hand, the one-push auto white balance control employs a white balance detection area WB₁ (FIG. 16A) which can be varied both in position and dimensions. Thus, the white balance detection area WB₁ can be located on a white part W₁ of an object in the scene in the field of view of the video camera, as on FIG. 16B, whereupon, in response to actuation or pushing of an auto white balance setting button (not shown), white balance control is carried out on the basis of a signal derived during scanning of the white balance detection area WB₁.

Since the position and dimensions of the white balance detection area WB₁ can be changed as desired under the control of the system controller 12, the white balance control can be effected with reference to a white portion of any object, such as, clothes or the like, appearing in the scene in the field of view of the camera. Therefore, it is not necessary to perform white balance adjustment with reference to a white cap or the like inconveniently placed over the lens. White balance control with reference to a white portion of an object in the field of view of the video camera ensures more precise white balance adjustment than that achieved when white balance is effected with reference to the integrated value of the entire picture which is assumed to be white.

In effecting the one-push auto white balance control as generally described above, a plurality of white balance detection areas may be set or defined, and then the one of those white balance detection areas whose shape and size approximate the shape and size of an area of the image of a white substantially as defined by a so-called black body radiation curve is selected for actual use in achieving white balance control.

As shown specifically on FIGS. 4A and 4B in the one-push automatic white balance detector 23 of a video camera embodying this invention, the luminance signal Y from the Y separator circuit 32 is supplied through a gate circuit 61A to an integrator circuit 62A. The chrominance signals C_(R) and C_(B) from the C separator circuit 33 are supplied through gate circuits 61B and 61C to integrator circuits 62B and 62C, respectively. Outputs of the integrator circuits 62A, 62B and 62C are supplied, as automatic white balance detection signals, to the system controller 12 through the serial output port 29. The gate circuits 61A, 61B and 61C are supplied with gate enabling signals from the automatic white balance detection area setting circuit 26 for defining each white balance detection area. In the fully automatic white balance control mode, the white balance detection area is set by the circuit 26 to be substantially coextensive with the image projected on the CCD imaging device 8. In the one-push auto white balance control mode, the location and dimensions of the white balance detection area WB are determined by the circuit 26 and are displayed to the camera operator, as on FIG. 16A, so that the operator can manipulate the camera for causing the displayed area WB₁, to correspond with a white part of the image projected on the CCD imaging device 8, as on FIG. 16B. Then, actuation of the white balance setting button suitably causes the gate enabling signals to be supplied from the circuit 26 to the gate circuits 61A, 61B and 61C.

The system controller 12 performs the following calculations on the integrated values IN(Y), IN(C_(R)) and IN(C_(B)) of the luminance signal Y and the chrominance signals C_(R) and C_(B), derived from the integrator circuits 62A, 62B and 62C, respectively.

By subtracting integrated values IN(C_(R)) and IN(C_(B)) of the chrominance signals C_(R) and C_(B) from the integrated value IN(Y) of the luminance signal Y, an integrated value IN(G) of the green (G) color signal is obtained as follows: ##EQU5##

By adding the integrated value IN(C_(R)) of the chrominance signal C_(R) and the integrated value IN(G) of the green (G) color signal obtained in the foregoing process, integrated value IN(R) of the red (R) color signal is obtained as follows:

    IN(C.sub.R)+IN(G)=IN(2R-G)+IN(G)=IN(2R)

By adding the integrated value IN(C_(B)) of the chrominance signal C_(B) and the integrated value IN(G) of the green (G) color signal obtained in the foregoing process, integrated value IN(B) of the blue (B) color signal is obtained as follows:

    IN(C.sub.B)+IN(G)=IN(2B-G)-IN(G)=IN(2B)

The integrated values of the levels of the three-component color signals R, G and B obtained, as above, in the system controller 12 are communicated through a connection 12a to the processor 10, and employed in the latter, as in a conventional white balance control circuit, for determining the gains of the respective three-component color signals R, G and B that may be derived from the luminance and chrominance signals at the output terminals 16A and 16B (FIG. 1) so as to maintain a predetermined ratio of the levels of the three-component color signals R, G and B.

Referring now to FIG. 17, it will be seen that, in a color video camera that may be otherwise similar to the camera described above with reference to FIGS. 1 and 4, and in which corresponding components are identified by the same reference numerals, the output of the CCD imaging device 2 representing a light image projected on the latter is supplied to a sample and hold circuit 7 operating at the pixel rate. The successive samples or pixels of the image output are supplied through the AGC circuit 8 to the A/D converter 9 which digitizes the image signal for each pixel. The output of the A/D converter is supplied to a camera signal processing circuit 10' which produces therefrom a digital luminance signal Y and digital color difference signals (R-Y) and (B-Y) for each pixel.

The digital color video signal components, that is, luminance signal Y and color difference signals (R-Y) and (B-Y) are supplied to gate circuits 61'A, 61'B and 61'C, respectively, and also to a white detector circuit 100 which may be constituted by a microprocessor. The white detector circuit 100 suitably determines from the digital luminance signal Y and digital color difference signals (R-Y) and (B-Y) for each pixel whether such pixel is nearly white or not.

More specifically, as shown in FIG. 18, a rectangular area, which is indicated in dotted lines at A, represents a range of deviation from white indicated at W on a black body radiation curve B. In response to each pixel which is determined by the white detecting circuit 100 to be within the area or range A, the white detecting circuit 100 provides a gate enabling signal to the gates 61'A, 61'B and 61'C to permit the digital luminance signal Y and the digital color difference signals (R-Y) and (B-Y) for such pixel to be supplied through the gate circuits to respective integrator circuits 62'A, 62'B and 62'C. Thus, the integrator circuits 62'A, 62'B and 62'C integrate or average only the values of the luminance signal and the color difference signals for the pixels which are within a predetermined range of white as defined by the black body radiation curve B. The average values or outputs of the integrator circuits 62'A, 62'B and 62'C are supplied to a controller 12' which suitably calculates therefrom gain setting signals supplied to the camera signal processing circuit 10' and employed in the latter for controlling the gains of the red signal R and the blue signal B relative to the green signal G so that the integrated values of pixels which are truly white will be displayed as being truly white no matter what source of illumination is employed.

Further, in accordance with this invention, the width of the rectangular area A on FIG. 18, that is, the range of the deviation from white of the pixels that cause the white detecting circuit 100 to open the gate circuits 61'A, 61'B and 61'C is initially relatively wide and, as the white balance adjustment is performed, the area A is narrowed, that is, the range is reduced for ensuring that, in the latter portion of a white-balance control operation, the integrated value of a pixel that is very nearly white is made to become truly white, as defined by the curve B.

It will be appreciated that, since only pixels that are nearly white are used for achieving the white-balance adjustment, accurate white-balance control is achieved regardless of the content of the scene or picture in the field of view of the camera. Further, the reduction of the area or range A from the beginning to the end of a white-balance control operation ensures that accurate white-balance will be achieved under any light source.

Although preferred embodiments of the present invention have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to such embodiments, and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims. 

What is claimed is:
 1. A color video camera comprising:image pickup means having a light reception surface for providing color video signal components corresponding to a light image projected on said surface; detecting means for detecting levels of said color video signal components which correspond only to at least substantially white regions of said light image including means for sampling said color video signal components to provide pixel-by-pixel values thereof and means for determining which of said pixel-by-pixel values are within a predetermined range of white as defined by a black body radiation curve means for receiving only said pixel-by-pixel values which are determined to be within said range and for integrating the received pixel-by-pixel values so as to obtain average values thereof as white-balance detection signals; and means for effecting white-balance control with reference to said white-balance detection signals derived from the color video components; said detecting means further including means for varying said range in dependence upon the deviation of said white-balance detection signals from a relationship characteristic of a desired white-balance.
 2. A color video camera according to claim 1; in which said color video signal components include luminance and chrominance components; said means for integrating includes an integrating circuit for each of said luminance and chrominance components; and said means for detecting includes a gate circuit for each said integrating circuit so that the latter receives the respective color video signal component only when the respective gate circuit is enabled.
 3. A color video camera comprising:image pickup means having a light reception surface for providing color video signal luminance and chrominance components corresponding to a light image projected on said surface; an integrating circuit for each of said luminance and chrominance components, each said integrating circuit providing an average value of the respective component; a gate circuit for each said integrating circuit permitting the latter to receive the respective color video signal component only when the respective gate circuit is enabled; means for determining, pixel-by-pixel, when said color video signal components have values that are within a predetermined range of white as defined by a black body radiation curve; means for enabling each said gate circuit whenever it is determined that said values are within said range; and means for effecting white-balance control with reference to said average values of the components; said range being relatively wide when said values of the color video signal components deviate substantially from a relationship characteristic of a desired white balance, and said range being reduced as said values of the color video signal components near said relationship characteristic of said desired white-balance.
 4. A method of generating white-balance detection signals from color video signal components corresponding to a light image projected on an image pickup of a color video camera, said method comprising the steps of:sampling said color video signal components to provide pixel-by-pixel values thereof; determining which of said pixel-by-pixel values are within a predetermined range of white as defined by a black body radiation curve; integrating only said pixel-by-pixel values which are determined to be within said range so as to obtain average values thereof as said white-balance detection signals; and varying said range in dependence upon the deviation of said white-balance signals from a relationship characteristic of a desired white-balance.
 5. A method of generating white-balance detection signals from color video signal components corresponding to a light image projected on an image pickup of a color video camera, said method comprising the steps of:determining, pixel-by-pixel, when said color video signal components have values that are within a predetermined range of white as defined by a black body radiation curve; gating the color video signal components to respective integrators whenever it is determined that said values are within said range; integrating the gated color video signal components so as to obtain average values thereof as said white-balance detection signals; making said range relatively wide when said values of the color video signal components deviate substantially from a relationship characteristic of a desired white-balance; and reducing said range as said values of the color video signal components near said relationship characteristic of said desired white-balance. 